Piston for an internal combustion engine

ABSTRACT

A piston for an internal combustion engine includes a piston head and a piston skirt, wherein the piston head has a piston base, a circumferential ring part and, in the region of the ring part, a circumferential closed cooling channel or sealed cavity. An inner side of the piston has two lower surfaces which transform continuously in the region of the piston central axis (M) to form an arched surface. The piston skirt has piston hubs provided with hub bores which are interconnected by means of running surfaces which have inner surfaces facing the inside of the piston. Starting from the free ends of the piston skirt, inside the piston on the pressure side (DS) and/or counter pressure side (GDS), an inner surface of a running surface continuously transforms into a guiding surface for a coolant which transforms continuously on the side thereof into a lower surface.

The present invention relates to a piston for an internal combustionengine, having a piston head and a piston skirt, wherein the piston headhas a piston crown, a circumferential ring belt, as well as acircumferential, closed cooling channel or a circumferential, closedcavity in the region of the ring belt, wherein an inside of the pistonhas two lower surfaces that make a constant transition into an archedsurface in the region of the center piston axis, wherein the pistonskirt has pin bosses provided with pin bores, which pin bosses areconnected with one another by way of working surfaces, which have innersurfaces that face the piston interior.

The piston of the stated type is a piston having spray cooling, i.e.cooling of the piston takes place by means of spraying it with coolantfrom the piston-skirt-side end. It has been shown that in pistons havinga small compression height, in particular, the coolant jet ispredominantly thrown back directly from the impact location. This hasthe result that a noticeable cooling effect is brought about at thislocation, but at other locations, sufficient cooling is not achieved.For this reason, it is observed, in the case of such pistons, that theybecome too hot during engine operation at greater loads, and do notwithstand long-term stress.

The task of the present invention consists in further developing apiston of the stated type in such a manner that more uniform spraycooling is achieved.

The solution consists in that an inner surface of the working surfacemakes a constant transition, proceeding from the free end of the pistonskirt in the piston interior, on the major thrust side and/or minorthrust side, into a guide surface for cooling oil, which in turn makes aconstant transition into a lower surface.

The piston according to the invention is characterized in that thesprayed-on coolant is not directly thrown back from the impact location.Instead, the coolant jet impacts the guide surface essentiallytangentially, and is guided in such a manner that it flows over thelower surface in the direction of the arched surface. The coolanttherefore flows back in the direction of the crankshaft with asignificant delay. In the end result, for one thing, a significantlygreater surface area on the inside of the piston is wetted with coolantand cooled, and for another thing, the coolant has a significantlylonger dwell time on the inside of the piston. This leads, in total, toa clearly stronger and more uniform cooling effect on the inside of thepiston. The heat transported to the inside of the piston from thedirection of the piston crown during engine operation is conducted awaysignificantly more effectively.

Advantageous further developments are evident from the dependent claims.

In an optimal case, the coolant is conducted by the guide surface insuch a manner that it flows over the lower surface in the direction ofthe arched surface and beyond that over the opposite lower surface andguide surface in the direction of the crankshaft. For this reason it ispreferred that the embodiment according to the invention is providedwith a guide surface both on the major thrust side and on the minorthrust side.

A preferred further development provides that a tangent applied in theregion of the inside, at an angle δ to the center piston axis, enclosesan angle β with the center piston axis, that each lower surface enclosesan angle α with the center piston axis, and that the angle β is lessthan or equal to the angle α. The resulting geometry of the guidesurface and of the lower surface allows particularly effective guidingof the sprayed-on coolant from the guide surface to the lower surfaceand a particularly advantageous coolant flow, in terms of flowtechnology. Particularly preferably, the tangent is applied in theregion of a guide surface.

A particularly preferred embodiment of the present invention consists inthat at least one bore closed toward the outside is provided, which boreis disposed between a working surface and a pin bore and opens into acoolant space, wherein the cavity and the at least one bore contain aheat transfer medium in the form of a metal having a low melting pointor a metal alloy having a low melting point. Metallic heat transfermedia bring about particularly effective cooling of the piston head andparticularly effective heat distribution.

In such pistons, the “cooling channel” that usually accommodates coolingoil is completely closed, i.e. neither inlet openings nor outletopenings for coolant are present. For this reason, the discussionhereinafter in connection with such pistons will refer not to a coolingchannel but rather to a closed cavity or, in short, a cavity.

In a piston filled with such a metallic heat transfer medium, the heattransfer medium cannot exit from the cavity. The heat absorbed by theheat transfer medium during engine operation, from the direction of thepiston crown, is given off directly to the surroundings, particularly tothe region of the ring belt and to the lower region of the cavity. Forthis reason, the configuration, according to the invention, of theinside of the piston is particularly preferred in connection with suchpistons. The heat absorbed by the heat transfer medium is transferred inthe direction of the inside of the piston and transported awayparticularly effectively by the sprayed-on coolant.

In such pistons, the maximal height of the cavity in the region of theworking surfaces is preferably greater than its maximal height in theregion of the pin bosses. As a result, the heat transferred by the heattransfer medium to the inside of the piston in the region of the workingsurface can be transported away particularly effectively by thesprayed-on coolant. In the region of the working surfaces, the wallthickness of the inside of the piston is so slight that effective heattransfer takes place, without impairing the stability of the piston. Thesymmetrically varying cross-section of the cooling channel furthermoreleads to the result that the piston according to the invention is betterbalanced and therefore can be better guided in the cylinder duringengine operation. Lower friction losses are found than those in thestate of the art.

A further advantageous embodiment of the piston according to theinvention provides that a lowermost ring groove having a groove heighth3 is provided within the ring belt, that a distance a is providedbetween the lower flank of the lowermost ring groove and the lowestpoint of the cavity, and that the distance a is equal to or greater thanthe groove height h3. The greater the difference between the grooveheight and the distance, the greater the maximal height of the cavity,and therefore the more effective the heat transfer from the heattransfer medium accommodated in the cavity to the inside of the piston.Furthermore, the greater the distance between the lower flank of thering groove and the lowest point of the cavity, the less heat istransferred to the region of the lowermost ring groove during engineoperation, so that the risk of carbonization is greatly reduced orcompletely prevented in this region.

Preferably, an outer wall of the cavity facing the ring belt, in thedirection of the piston crown, is configured to be inclined, at least inpart, toward the center piston axis. As a result, the movement of theheat transfer medium accommodated in the cavity, brought about by whatis called the “Shaker effect” is optimized during the piston strokeduring engine operation. Furthermore, too much heated heat transfermedium is prevented from coming into contact with the outer wall andexcessively heating the ring belt, so that the risk of carbonization inthe region of the ring grooves is prevented.

It is practical if the inclined outer wall of the cavity encloses anangle of 1° to 10° with an axis parallel to the center piston axis. Inthis way, the cavity is additionally prevented from being excessivelyconstricted, and an effective heat transfer effect is maintained.

Preferably, the fill amount of the heat transfer medium amounts to 5% to10% of the total volume of the cavity and of the at least one bore. Thishas the advantageous effect that the metallic heat transfer mediumtransports the heat more effectively into the lower region of thecavity, in the direction of the piston skirt, so that less heat is givenoff in the direction of the ring belt.

In this piston type, it is particularly advantageous to configure anouter wall of the cavity facing the ring belt, in the direction of thepiston crown, to be inclined, at least in part, toward the center pistonaxis, in order to prevent excessive heating, as it is otherwise observedin this piston type.

Metals having a low melting point, which are suitable for use ascoolants, are, in particular, sodium or potassium. In particular,Galinstan® alloys, bismuth alloys having a low melting point, andsodium-potassium alloys can be used as metal alloys having a low meltingpoint.

Alloy systems composed of gallium, indium, and tin, which are liquid atroom temperature, are called Galinstan® alloys. These alloys consist of65 wt.-% to 95 wt.-% gallium, 5 wt.-% to 26 wt.-% indium, and 0 wt.-% to16 wt.-% tin. Preferred alloys are those, for example, having 68 wt.-%to 69 wt.-% gallium, 21 wt.-% to 22 wt.-% indium, and 9.5 wt.-% to 10.5wt.-% tin (melting point −19° C.), 62 wt.-% gallium, 22 wt.-% indium,and 16 wt.-% tin (melting point 10.7° C.), as well as 59.6 wt.-%gallium, 26 wt.-% indium, and 14.4 wt.-% tin (ternary eutectic, meltingpoint 11° C.).

Bismuth alloys having a low melting point are known in great numbers.These include, for example, LBE (eutectic bismuth-lead alloy, meltingpoint 124° C.), Rose's metal (50 wt.-% bismuth, 28 wt.-% lead, and 22wt.-% tin, melting point 98° C.), Orion metal (42 wt.-% bismuth, 42wt.-% lead, and 16 wt.-% tin, melting point 108° C.); quick solder (52wt.-% bismuth, 32 wt.-% lead, and 16 wt.-% tin, melting point 96° C.),d'Arcet's metal (50 wt.-% bismuth, 25 wt.-% lead, and 25 wt.-% tin),Wood's metal (50 wt.-% bismuth, 25 wt.-% lead, 12.5 wt.-% tin, and 12.5wt.-% cadmium, melting point 71° C.), Lipowitz' metal (50 wt.-% bismuth,27 wt.-% lead, 13 wt.-% tin, and 10 wt.-% cadmium, melting point 70°C.), Harper's metal (44 wt.-% bismuth, 25 wt.-% lead, 25 wt.-% tin, and6 wt.-% cadmium, melting point 75° C.), Cerrolow 117 (44.7 wt.-%bismuth, 22.6 wt.-% lead, 19.1 wt.-% indium, 8.3 wt.-% tin, and 5.3wt.-% cadmium, melting point 47° C.), Cerrolow 174 (57 wt.-% bismuth, 26wt.-% indium, 17 wt.-% tin, melting point 78.9° C.), Field's metal (32wt.-% bismuth, 51 wt.-% indium, 17 wt.-% tin, melting point 62° C.), andWalker's alloy (45 wt.-% bismuth, 28 wt.-% lead, 22 wt.-% tin, and 5wt.-% antimony).

Suitable sodium-potassium alloys can contain 40 wt.-% to 90 wt.-%potassium. The eutectic alloy NaK with 78 wt.-% potassium and 22 wt.-%sodium (melting point −12.6° C.) is particularly suitable.

The heat transfer medium can additionally contain lithium and/or lithiumnitride. If nitrogen is used as a protective gas during filling, it canreact with the lithium to form lithium nitride and can be removed fromthe cavity in this manner.

The heat transfer medium can furthermore contain sodium oxides and/orpotassium oxides, if any dry air that might be present has reacted withthe heat transfer medium during filling.

Preferably, four bores are provided, which are disposed between aworking surface and a pin bore, in order to achieve a particularlyuniform temperature distribution in the piston.

It is practical if the at least one bore is closed off by means of aclosure element, in order to prevent the heat transfer medium fromexiting. The closure element can be provided at the free end of thepiston skirt. Preferably, the closure element is provided in the pistoncrown, in order to be able to conveniently fill the cavity and the atleast one bore.

An exemplary embodiment of the present invention will be explained ingreater detail below, using the attached drawings. These show, in aschematic representation, not true to scale:

FIG. 1 an exemplary embodiment of a piston according to the invention,in section;

FIG. 2 the piston according to FIG. 1 in a perspective representation,in section;

FIG. 3 the piston according to FIG. 1 in section through two bores thatlie diagonally opposite one another.

FIGS. 1 to 3 show an exemplary embodiment of a piston 10 according tothe invention. The piston 10 can be a one-part cast piston or amulti-part joined piston. The piston 10 can be produced from aniron-based material and/or from a light metal material. The piston 10according to the exemplary embodiment shown in FIGS. 1 to 3 is filledwith a metallic heat transfer medium, as described above. Heat transfermedia that are solid and can be kneaded at room temperature, for examplesodium, are preferred.

FIGS. 1 to 3 show a two-part joined box piston 10 as an example. Thepiston 10 has a piston head 11 having a piston crown 13 having acombustion bowl 14, a circumferential top land 15, and a circumferentialring belt 16 having ring grooves 17 a, 17 b, 17 c for accommodation ofpiston rings (not shown). At the level of the ring belt 16, acircumferential, closed cavity 18 is provided, which does not have anyinlet or outlet openings.

The piston 10 furthermore has a piston skirt 21 having pin bosses 22 andpin bores 23 for accommodation of a piston pin (not shown). The pinbosses 22 are connected with the piston head 11 in known manner, by wayof pin boss connections. The pin bosses 22 are connected with oneanother by way of working surfaces 24, 25.

In the present exemplary embodiment, the piston 10 is composed of apiston base body 10 a and a piston ring element 10 b, which are producedin known manner by means of forging or casting, pre-machined, and joinedby means of a welding method, particularly a laser welding method,thereby resulting in circumferential weld seams 10 c, 10 d. The piston10 can, of course, also be joined together in known manner, from anupper piston part that comprises the piston head 11 and a lower pistonpart that comprises the piston skirt 21, for example. The piston 10 canalso be configured as a one-part piston that is cast in known manner,whereby salt cores, for example, are used to form the cavity 18 and thebores 25 (see below).

In the exemplary embodiment, the piston 10 has four bores 26 (see, inparticular, FIGS. 2 and 3). The bores 26 run approximately axially andparallel to the center piston axis M in the exemplary embodiment. Thebores 26 can, however, also run inclined at an angle relative to thecenter piston axis M (not shown). The bores 26 are disposed between aworking surface 24, 25 and a pin bore 23. The bores 26 open into thecavity 18. The coolant space 18 and the bores 26 are filled with ametallic heat transfer medium 27, sodium in the exemplary embodiment.

The size of the bores 26 and the fill amount of the heat transfer medium27 are based on the size and the material of the piston 10. The coolingoutput can be controlled by way of the amount of the heat transfermedium 27 added, taking its heat conductivity coefficient intoconsideration. The fill amount should preferably amount to 5% to 10% ofthe total volume of the cavity 18 and of the bores 26. In this case, theknown Shaker effect can be additionally utilized for particularlyeffective heat distribution in the piston 10 during operation. Forsodium as a heat transfer medium 27, with a temperature during operationof maximally 350° C., a maximal surface temperature of the piston 10 ofabout 260° C. occurs at a cooling output of 350 kW/m².

During engine operation, the inside 12 of the piston 10 according to theinvention is cooled by means of spray cooling. For this purpose, aspray-on nozzle 30 for a coolant is provided in the engine, in knownmanner (see FIG. 1), which nozzle is provided fixed in place on thecrankcase, for example.

Of course, the piston according to the invention can also have aconventional cooling channel for cooling oil, which has inlet and outletopenings for the cooling oil. In the case of such a piston, as well,improved spray cooling in the region of the inside 12 is achieved bymeans of the configuration of the inside 12 of the piston 10 accordingto the invention.

To improve the cooling effect of the spray cooling, in this exemplaryembodiment it is provided that the maximal height h1 of the cavity 18 inthe region of the working surfaces 24, 25 is greater than the maximalheight h2 in the region of the pin bosses 22 (see FIG. 2). This bringsabout the result that the heat transfer medium 27 conducts the heattransported by way of the cavity 18 from the direction of the pistoncrown 13, in the region of the working surfaces, away particularlyeffectively in the direction of the inside 12 of the piston 10. In theregion of the working surfaces 24, 25, the wall thickness of the wallregion 31 between the cavity 18 and the inside of the piston is soslight that effective heat transfer to the inside 12 of the piston 10takes place, without impairing the stability of the piston 10. Thesymmetrically varying cross-section of the cavity 18 furthermore leadsto the result that the piston 10 according to the invention is betterbalanced and therefore can be better guided in the cylinder duringengine operation. Lower friction losses are found than those in thestate of the art.

Furthermore, in the present exemplary embodiment it is provided that thelowermost ring groove 17 c has a groove height h3, and that the grooveheight h3 is less than or equal to the distance a between the lowerflank of the lowermost ring groove 17 c and the lowest point of thecavity 18. The greater the difference between the groove height and thedistance, the greater the maximal height of the cavity, and thereforethe more effective the heat transfer from the heat transfer mediumaccommodated in the cavity to the inside of the piston. Furthermore, thegreater the distance between the lower flank of the ring groove and thelowest point of the cavity, the less heat is transferred to the regionof the lowermost ring groove during engine operation, so that the riskof carbonization is greatly reduced or completely prevented in thisregion.

Furthermore, it is provided, according to the invention, that proceedingfrom the free end of the piston skirt 21, in the piston interior, afirst inner surface 32 a of a working surface 24 makes a constanttransition into a first guide surface 33 a for coolant on the majorthrust side DS and/or minor thrust side GDS, which guide surface in turnmakes a constant transition into a first lower surface 34 a. The firstlower surface 34 a in turn makes a constant transition into an archedsurface 35, which is disposed in the region of the center piston axis M.

In the exemplary embodiment shown, it is furthermore provided that thearched surface 35 in turn makes a constant transition into a secondarched lower surface 34 b, which in turn makes a transition into asecond guide surface 33 b for coolant, which opens into a second innersurface 32 b of a working surface 25, in constant manner.

The two inner surfaces 32 a, 32 b of the two guide surfaces 33 a, 33 b,the two lower surfaces 34 a, 34 b, and the arched surface 35 form theinside 12 of the piston 10.

This embodiment according to the invention brings about the result thatthe sprayed-on coolant is not directly thrown back from the impactlocation. Instead, the coolant jet impacts the guide surface 33 aessentially tangentially, and is guided in such a manner that thecoolant flows over the lower surface 34 a in the direction of the archedsurface 35. In an optimal case, the coolant flows back in the directionof the crankshaft from the arched surface 35, by way of the lowersurface 34 b, the guide surface 33 b, and the inner surface 32 b.

The coolant therefore flows back in the direction of the crankshaft witha significant delay. In the end result, for one thing, a significantlygreater surface area on the inside 12 of the piston 10 is wetted withcoolant and cooled, and for another thing, the coolant has asignificantly longer dwell time on the inside 12 of the piston 10. Thisleads, in total, to a clearly stronger and more uniform cooling effecton the inside of the piston. The heat transported in the direction ofthe inside 12 of the piston 10 from the direction of the piston crown13, by way of the cavity 18 and the combustion bowl 14, is conductedaway significantly more effectively.

The exemplary embodiment shown is furthermore characterized in that thetangent T applied in the region of the inside 12, at an angle δ to thecenter piston axis M, encloses an angle β with the center piston axis M.Furthermore, each lower surface 34 a, 34 b encloses an angle α with thecenter piston axis M. In this connection, the angle β is less than orequal to the angle α. The resulting geometry of the guide surfaces 33 a,33 b and of the lower surfaces 34 a, 34 b allows particularly effectiveguiding of the sprayed-on coolant from the guide surface 33 a to thelower surface 34 a, as well as a particularly advantageous coolant flow,in terms of flow technology.

In the present exemplary embodiment, it is furthermore provided that anouter wall 36 of the cavity 18 facing the ring belt 16, in the directionof the piston crown 13, is configured to be inclined, at least in part,toward the center piston axis M. In the present case, the inclined outerwall 36 of the cavity 18 encloses an angle γ of preferably 1° to 10°with an axis A parallel to the center piston axis M. This configurationbrings about the result that the ring belt 16 is not excessively heated,and the risk of carbonization at the ring grooves is prevented. Thiseffect is essentially based on the following mechanisms. As the resultof the Shaker effect during engine operation, heated heat transfermedium 27 from the direction of the piston crown 13 is essentially movedvertically downward during the upward stroke of the piston 10. This hasthe result that contact between the outer wall 36 of the cavity 18 andthe hot heat transfer medium 27 is prevented, to the greatest possibleextent. The heat transfer medium 27 therefore gives off a significantpart of its heat when it first impacts the spray-cooled of the cavity 18in the direction of the inside 12 of the piston 10. The heat transfermedium 27, which is now less hot, can flow along the outer wall 36 ofthe cavity 18 in the direction of the piston crown 13, withoutexcessively heating the ring belt 16. Furthermore, the outer wall 36 ofthe cavity 18 is configured to be thickened in the region of the ringbelt 16, so that the passage of heat in the direction of the ring belt16 is additionally reduced.

For production of the piston 10, a piston base body 10 a and a pistonring element 10 b are produced by means of forging or casting andpre-machined. Then, the metallic heat transfer medium 27, which is solidand can be kneaded at room temperature, is placed into the region of thepiston base body 10 a, which forms part of the cavity 18 in the finishedpiston 10 (see FIG. 1). Then the piston base body 10 a and the pistonring element 10 b are put together and joined by means of a weldingmethod, for example laser welding, and firmly connected with oneanother, thereby resulting in circumferential weld seams 10 c, 10 d.

If a one-part piston is supposed to be produced or if a metallic heattransfer medium that is liquid at room temperature is used, a fillingopening 37, 38 must be present. This filling opening can be providedeither at the free end of the piston skirt 21 (filling opening 37 inFIG. 2) or in the piston crown 13 (filling opening 38 in FIG. 1). Thefilling opening is tightly sealed after filling with the heat transfermedium has taken place, by means of a closure element (closure element41 in FIG. 2 or closure element 42 in FIG. 3). The closure element 41,42 can be configured, for example, as a pressed-in steel ball, awelded-on lid, or a pressed-in cap.

To fill the piston 10 with a liquid heat transfer medium, a lance isintroduced through the filling opening 37, 38, and flushed by means ofnitrogen or by means of another suitable inert gas or by means of dryair. To introduce the heat transfer medium 27, this medium is guidedthrough the filling opening 37, 38 under protective gas (for examplenitrogen, inert gas or dry air), so that the heat transfer medium 27 isaccommodated in the bores 26 and/or in the cavity 18.

A further method for filling the piston 10 is characterized in thatafter flushing with nitrogen, inert gas or dry air, the bores 26 and thecavity 18 are evacuated, and the heat transfer medium 27 is introducedin a vacuum. As a result, the heat transfer medium 27 can move back andforth in the cavity 18 and in and out in the bores 26 more easily,because it is not hindered by protective gas that is present.

Another possibility for removing the protective gas from the cavity 18and the bores 26 consists in using nitrogen or dry air (i.e. essentiallya mixture of nitrogen and oxygen) as the protective gas and adding asmall amount of lithium to the heat transfer medium 27, empiricallyabout 1.8 mg to 2.0 mg lithium per cubic centimeter of gas space (i.e.volume of the cavity 18 plus volume of the bores 26). While sodium andpotassium, for example, react with oxygen to form oxides, the lithiumreacts with nitrogen to form lithium nitride. The protective gas isthereby practically bound completely in the heat transfer medium 27, asa solid.

1. Piston (10) for an internal combustion engine, having a piston head(11) and a piston skirt (21), wherein the piston head (11) and a pistoncrown (13), a circumferential ring belt (16), as well as acircumferential, closed cooling channel or closed cavity (18) in theregion of the ring belt (16), wherein an inside (12) of the piston (10)has two lower surfaces (34 a, 34 b) that make a constant transition intoan arched surface (35) in the region of the center piston axis (M),wherein the piston skirt (21) has pin bosses (22) provided with pinbores (23), which pin bosses are connected with one another by way ofworking surfaces (24, 25), which have inner surfaces (32 a, 32 b) thatface the piston interior, wherein proceeding from the free end of thepiston skirt (21), in the piston interior, an inner surface (32 a, 32 b)of a working surface (24, 25) makes a constant transition into a guidesurface (33 a, 33 b) for coolant, on the major thrust side (DS) and/oron the minor thrust side (GDS), which guide surface in turn makes aconstant transition into a lower surface (34 a, 34 b).
 2. Pistonaccording to claim 1, wherein proceeding from the free end of the pistonskirt (21), in the piston interior, an inner surface (32 a, 32 b) of aworking surface (24, 25) makes a constant transition into a guidesurface (33 a, 33 b) for coolant, both on the major thrust side (DS) andon the minor thrust side (GDS), which guide surface in turn makes aconstant transition into a lower surface (34 a, 34 b).
 3. Pistonaccording to claim 1, wherein a tangent (T) applied in the region of theinside (12), at an angle (δ) to the center piston axis (M), encloses anangle (β) with the center piston axis (M), wherein each lower surface(34 a, 34 b) encloses an angle (α) with the center piston axis (M), andwherein the angle (β) is less than or equal to the angle (α).
 4. Pistonaccording to claim 3, wherein the tangent (T) is applied in the regionof a guide surface (33 a, 33 b).
 5. Piston according to claim 1, whereinat least one bore (26) closed toward the outside is provided, which boreis disposed between a working surface (24, 25) and a pin bore (23) andopens into the cavity (18), wherein the cavity (18) and the at least onebore (26) contain a heat transfer medium (27) in the form of a metalhaving a low melting point or a metal alloy having a low melting point.6. Piston according to claim 5, wherein the maximal height (h1) of thecavity (18) in the region of the working surfaces (24, 25) is greaterthan its maximal height (h2) in the region of the pin bosses (22), 7.Piston according to claim 5, wherein a lowermost ring groove (17 c)having a groove height (h3) is provided within the ring belt (16),wherein a distance (a) is provided between the lower flank of thelowermost ring groove (17 c) and the lowest point of the cavity (18),and wherein the distance (a) is equal to the groove height (h3) orgreater than the groove height (h3).
 8. Piston according to claim 5,wherein an outer wall (36) of the cavity (18) facing the ring belt (16),in the direction of the piston crown (13), is configured to be inclined,at least in part, toward the center piston axis (M).
 9. Piston accordingto claim 8, wherein the inclined outer wall (36) of the cavity (18)encloses an angle (γ) of 1° to 10° with an axis (A) parallel to thecenter piston axis (M).
 10. Piston according to claim 5, wherein thefill amount of the heat transfer medium (27) amounts to 5% to 10% of thetotal volume of the cavity (18) and of the at least one bore (26). 11.Piston according to claim 5, wherein sodium or potassium is contained asa metal having a low melting point.
 12. Piston according to claim 5,wherein the metal alloy having a low melting point is selected from thegroup comprising Galinstan® alloys, bismuth alloys having a low meltingpoint, and sodium-potassium alloys.
 13. Piston according to claim 5,wherein four bores (26) are provided, which are disposed between aworking surface (24, 25) and a pin bore (23).
 14. Piston according toclaim 5, wherein the at least one bore (26) is closed off by means of aclosure element (41, 42).
 15. Piston according to claim 14, wherein theclosure element (41) is disposed at the free end of the piston skirt(21).
 16. Piston according to claim 14, wherein the closure element (42)is disposed in the piston crown (13).